National MP Sam Uffindell asked to leave Auckland's King's College after violent nighttime attack on younger student

National's newest MP Sam Uffindell was asked to leave a prestigious Auckland private school over 20 years ago after a violent nighttime attack on a younger student, it's been revealed.

Uffindell said he informed the party before he was selected as a candidate and went on to win the Tauranga by-election.

It emerged on Monday afternoon after reports Uffindell's victim had come forward.

The victim said the National MP had contacted him last year to make amends. He'd accepted that apology until he saw Uffindell was running for Parliament and felt the apology wasn't genuine.

The victim was only 13 when the attack happened and it involved Uffindell and three other teens at King's College.

Uffindell said in a statement: "It's one of the stupidest things I've ever done and something I deeply regret.

"I've worked hard to be a better person than I was as a teenager and to be a good role model to my children," he continued.

It was only last week in his maiden speech that Uffindell highlighted the need to address violence.

"Our region is beset by gang issues and, unfortunately, like the rest of New Zealand, a growing culture of lawlessness, lack of accountability, a sense of impunity, and significant underlying generational social problems," he said.

"We need friends, family, and, in particular, parents, to step up and show what is right."

Its selection process has been a problem for the National Party in recent years, prompting a review of how wannabe MPs are vetted.

There were the high-profile departures of MPs Hamish Walker, who leaked COVID-19 patient details to media, and Andrew Falloon, who sent unsolicited sexually explicit images to young women.

There was also Jake Bezzant, who left the party last year after serious allegations emerged from an ex-girlfriend.

On Monday, Prime Minister Jacinda Ardern said it was the responsibility of each party leader to ensure they choose appropriate candidates.

"Where there are issues, to the best of our ability we do draw those out so that we can be aware and that may mean that someone is not selected, but it may mean that we work to ensure there is a level of transparency around those issues."

In a statement to Newshub, the National Party said it "was proactively informed about this incident" by Uffindell during the selection process.

"It was a significant event reflecting a serious error of judgement by a then 16-year-old for which he has apologised and regrets to this day."